Transient control solution for optical networks

Abstract

An optically amplified wavelength division multiplexing network has the functionality to add / drop channels at the optical add / drop multiplexing (OADM) nodes. The OADM node includes a receiver amplifier, an OADM module, and a transmitter amplifier. Once the OADM node detects a loss of signal (LOS) due to a fiber cut or network element failure upstream, the receiver amplifier is kept in operation as a noise source. The output of the receiver amplifier is immediately raised by increasing pump power to compensate for the LOS. The noise power received at the transmitter amplifier from the receiver amplifier is substantially equal to the signal power expected before LOS. The transient effect of downstream optical amplifiers is therefore completely suppressed and the inter-channel stimulated Raman scattering (SRS) induced spectrum tilt does not change. After the noise power is raised, the receiver amplifier may be shut down at a speed much slower than the speed of downstream amplifier control circuitry.

Description

CROSS REFERENCE TO RELATED APPLICATIONS[0001]This application is a continuation of application Ser. No. 10 / 803,062, filed on Mar. 17, 2004, and claims benefit of U.S. Provisional Application. Ser. No. 60 / 503,959 filed on Sep. 18, 2005.FIELD OF THE INVENTION[0002]The invention relates to the field of high-speed optical telecommunication networks, and more particularly to a method and apparatus for suppressing the transient effect in surviving optical channels in the event of a fiber cut or network node failure.BACKGROUND OF THE INVENTION[0003]In optical telecommunication networks that use wavelength division multiplexing (WDM), multiple optical channels are carried on a single optical fiber. The single optical fiber is included in an optical fiber link, which includes the optical fiber and optical amplifiers and any other optical components between two connecting points or nodes along an optical transmission line. Each channel operates at a different wavelength in the optical fiber. ...

AI Technical Summary

Benefits of technology

"The present invention is a method and apparatus for suppressing the transient effect and related inter-channel stimulated Raman scattering (SRS)-induced spectrum tilt in an optically amplified wavelength division multiplexing (WDM) network. The invention addresses the need for a simple, low-cost, high-performance solution to effectively suppress these issues. The invention includes a receiver amplifier, an optical add / drop multiplexing module, and a transmitter amplifier. The receiver amplifier receives an optical signal with optical channels from an upstream optical fiber link and includes a pump laser. The optical add / drop multiplexing module drops some optical channels, passes through the remaining channels, and adds additional optical channels. The transmitter amplifier amplifies the passed optical channels and the added optical channels. The receiver amplifier also operates as a noise source in response to a loss of signal in the upstream optical fiber link. The method includes the steps of configuring the optical add / drop multiplexing node, calculating the target level of output noise power, and adjusting the pump power to achieve the target level of output noise power. The invention can provide a more effective solution for suppressing these issues in optical networks."

Problems solved by technology

The slow gain dynamics, however, also result in a slow response time.

In the presence of a large instantaneous change in input power to the EDFA, the slow response time can cause adverse effects downstream.

In an extreme case, a fiber cut or equipment failure upstream from the OADM may occur, causing a transient effect on the transmission of surviving channels in the downstream link, i.e., in those channels added at the OADM.

Due to the transient effect, the surviving channels may also experience transmission errors and, if severe enough, these errors may cause unwanted protection switching in optical networks.

Though the pump control method may be adequate to correct slow or small fluctuations in amplifier gain over time, it is inadequate to handle transient effects on surviving channels resulting from fiber cuts or equipment failure upstream from the amplifier.

In other words, the slow dynamics of EDFA, though advantageous in that the gain is subsequently undisturbed by the modulation of the transmission signal, pose a problem in the event of a fiber cut or equipment failure.

The pump control method is particularly inadequate in the case of the large channel count drop that occurs when there is a fiber cut or equipment failure in an optically amplified WDM network.

Two problems arise in this case.

A spectral power distribution as well as the optical power at the input to the downstream optical amplifier changes, therefore, when fiber cuts or equipment failures occur.

First, the sudden large drop in optical input power to an amplifier downstream of the OADM cannot be accommodated by the pump control method due to the slow gain dynamics of the EDFA.

Second, the large change in the spectral power distribution results in a large change of stimulated Raman scattering (SRS) induced spectrum tilt in the downstream transmission fiber not compensated for by the pump control method.

Consequently, both the transient gain and SRS spectrum tilt change could cause transmission errors for the channels transmitted to a downstream optical fiber link and undesired protection switching in the optical network.

The control speed of VOA, however, cannot match the spectrum tilt change and the algorithm to simultaneously control both the pump power and VOA is very complicated and inadequately slow.

Using multiple filler lasers occupying several WDM wavelength slots, therefore, reduces transmission capacity.

The phenomenon ultimately limits the maximum optical power that can be launched into the fiber.

Moreover, inhomogeneous broadening of optical amplifiers and the resulting spectral hole burning can cause gain variations at the optical wavelength of the filler laser, additionally limiting the capability of the filler laser control technique.

As a result, the filler laser control method is complicated, costly, inefficient, and requires a reduction of the total transmission capacity of the optical network.

In summary, these and other conventional methods which have been employed in an attempt to mitigate the transient effect are complex, costly, and largely incapable of error free performance.

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